Hi Ryan,

I think it would be a good idea to talk with a personal trainer at your gym so that you can best develop a weight-lifting program that is personally structured and best fits in with your overall marathon training.  (There are many variations, intensity levels, recommendations and so on).

I have really benefited from weight lifting, especially regarding the importance of upper-body strength for longer distances.  It was very helpful to work with a trainer initially to develop a plan that was specifically designed for the goal of improving running.  Depending on your schedule and expected outcomes, lifting 2 to 3 times per week might be of great assistance.  I think it would improve your performance (over an extended period of time, provided you were following a plan that compliments your marathon training). 

I have been lifting for several years now and it does make a difference with the right combination of other elements. 

Wishing you all the best.